So last week I talked about tracking and finding a high altitude weather balloon
using information transmitted on Amateur Radio using the Automatic Packet Reporting
System (APRS).

Integral to this effort was the use of the Global Positioning System. There
are 24 GPS satellites in known orbits at 12,000 miles above the earth transmitting
time information. GPS receivers measure the time difference between when a signal
is transmitted and received to determine distance. Using distances from multiple
satellites, the GPS receiver can can triangulate its position.

The GPS satellite constellation was in place by 1994. The signal available
to civilians initially had an intentionally produced inaccuracy called "selective
availability" but it was turned off in 2000.

At that time, a GPS enthusiast got the idea to hide a plastic bucket full of
"treasure" and post the coordinates on the Internet for other GPS
enthusiasts to find. Thus was born the "Great American GPS Stash Hunt,"
which is now known as Geocaching.

According to geocaching.com, there are 2,785,533 active geocaches and over
15 million geocachers worldwide. Geocaching has been popular for a long time,
but the numbers really exploded when smart phones with GPS receivers became
available.

As a minimum,a geocache is a container with a log book. It might contain also
items for trade or trackable items that are moved from cache to cache.

Geocachers finding a cache sign the log and post their find on the web site.

Current GPS receiver accuracy is about 20 feet or less, but various factors
can degrade the signal. Sometimes a GPS receiver will lead you right to a cache,
but the combined error of hider and searcher measurements could be significant.

Geocaches could be in plain sight, but most likely they are hidden. They could
be any size. It could take some time to find one even when you are very close.

Geocache descriptions provide difficulty ratings, so you know about how hard
a cache will be to get to and how hard it will be to find once you get close.
Some caches are "park and grabs" but others require a serious hike
or a lot of searching. They can be mundane hides or they can take you to places
you never know existed, even close to home.

Along with physical caches, there are geocaching events, which are opportunities
to "meet and greet" as well as go looking for local caches with other
geocachers. Attending an event is an ideal way to learn how to play the game.

There has been a "Mid-Winter Geocaching Get-Together" event in the
Adirondacks since 2006. This year it will be March 12 in the Speculator area.
You can see the details by looking up GC68A1T on geocaching.com.

You will need a geocaching.com account to view cache descriptions. A basic
account is free, requiring only that you provide an email address and choose
a user name and password.

If you like the outdoors, like puzzles and games, and like playing with gadgets,
geocaching may be for you. I highly recommend it.